1. Field of the Invention
The present invention relates to a recognition method of recognizing a plurality of non-contact IC cards and a recognition system for implementing that method.
2. Description of the Related Art
So far, a so-called polling selective method has been employed for communication between IC cards that transfer data using electromagnetic induction and the like and a reader/writer that recognizes the IC cards. The polling-selective method is a communication method that establishes a one-to-one correspondence between the reader/writer and the IC cards so that the reader/writer can sequentially poll the IC cards.
In the above polling selective method, a plurality of non-contact IC cards simultaneously exist in the communication area of one reader/writer. If more than one IC cards transmit response signals to the reader/writer in answering the reader/writer's polling trial, then these response signals collide, so that neither of the non-contact IC cards can perform normal communication with the reader/writer.
In order to solve the above problem that occurs from the simultaneous issuing of response signals by a plurality of IC cards in answering the reader/writer's polling trial, a radio recognition apparatus was disclosed in Japanese Patent Laid-Open Publication No. 9-6934. The radio recognition apparatus comprises recognition cards (equivalent to non-contact IC cards), each of which has a random-number generator and issues a response signal after a delay time determined by a random number generated by the random-number generator, and a transmitting/receiving decoder (equivalent to reader/writer), which recognizes the recognition cards.
However, in this radio recognition apparatus, the delay time until a recognition card transmits a response signal after receiving the reader/writer's polling trial is determined by a random number. Therefore, even if only one recognition card exists, so that the response signal could be sent immediately, the response signal cannot be sent until the delay time determined by the random number passes, so that the response characteristic deteriorates.
Also, a multi-access method for non-contact cards was disclosed in Japanese Patent Laid-Open Publication No. 4-362797. In this multi-access method, each child authority (non-contact card) that has received a read command issued from the parent authority examines a time sharing area addressed in the read command. If a temporary ID of its own is not in the area, then the child authority transmits its temporary ID to an empty location of the time sharing area determined by a generated random number to let the parent authority recognize the ID. In this way, the parent authority can simultaneously perform communication with a plurality of its child authorities in time sharing.
However, in the above multi-access method for non-contact cards, each child authority needs an extra circuit for recognizing a temporary ID sent from the parent authority, so that the apparatus becomes complex with higher costs.